The present invention generally relates to modifications for jet aircraft of the type having aft fuselage-mounted engine nacelles which overlap the trailing edge portions of the aircraft's wings. In particular, the present invention is directed to aircraft modifications adaptable for use with Gates Learjet type aircraft, especially Models 23, 24 and 25, by providing an essentially subsonic aerodynamic flow path through a channel area defined by the fuselage, wing, pylon and attached engine nacelle, during flight in the transonic regime, i.e. 0.75.ltoreq.M.gtoreq.0.83. The essentially subsonic air flow eliminates the possibility of a shock wave forming in the channel during high speed flight. The present invention is further directed to wing modifications for increasing the lift generated by the sustaining airfoil, while reducing drag generated during operation of the aircraft.
Extensive tests of an unmodified Gates Learjet type aircraft (Model 24, Serial Number 150) have shown that increased drag and the corresponding reduction in lift generated during transonic flight (M.gtoreq.0.75) of this aircraft tend to adversely increase its rate of fuel consumption. Furthermore, the adverse combination of increased drag and reduced lift contribute to the difficulty in maintaining level flight at altitudes of 50,000 feet or more M.S.L. as reported in Professional Pilot Magazine, March 1978, pages 40-44.
Applicant has determined that a major cause of increased drag for an unmodified Gates Learjet type aircraft operating in the transonic regime can be attributed to a converging-diverging channel defined by the fuselage, trailing edge portion of the wing, and the overlapping portion of the engine nacelle and pylon assembly. Air is caused to accelerate over the leading edge of the wing surface in a conventional manner, generating lifting forces for sustaining flight of the aircraft. As the accelerated air passing over an inboard portion of the wing enters the channel, it attempts to maintain a continuum in spite of the changing shape of the channel. This causes the air to accelerate until it reaches and exceeds the speed of sound, or M=1, substantially at the position of the channel throat, at which time a shock wave is generated within the converging-diverging channel by separation of the highly accelerated air. The shock wave acts as a plug within the channel, diverting upstream air around the outer surfaces of the nacelle and pylon assembly, creating turbulent air flow about the nacelle pylon assembly which greatly increases the level of drag generated during transonic flight of the aircraft.
One solution disclosed herein is to re-design the converging-diverging channel to entirely converge, rather than diverge through an aft portion. This solution is not completely effective, however, in that an entirely convergent channel will, itself, generate an increased amount of drag by effectively increasing the frontal area of the aircraft.
Applicant's preferred solution to the channel drag problem recognizes the importance of retaining a converging-diverging channel, while at the same time preventing the formation of a shock wave therein. Specifically, applicant has modified the original diverging channel to reduce, but not eliminate, the degree of divergence.
Conveniently, in a preferred embodiment of the present invention, the lower surface of the nacelle pylon assembly, which defines a portion of the channel, has been modified to reduce the channel divergence while maintaining substantially smooth, continuous flow lines therethrough. In addition, a cheek surface portion of the engine nacelle forming a wall of the channel has been reduced in diameter to increase the throat area of the channel and thus decrease the overall degree of divergence.
The standard Gates Learjet type wing can be derived from the NACA 64A-109 airfoil section, wherein: "6" designates the NACA series; "4" designates a chord-wise position of minimum pressure expressed in tenths of the chord; "A" designates an airfoil that is substantially straight from about eighttenths of the chord to the trailing edge; "1" designates a design lift coefficient in tenths; and "09" expresses the thickness of the wing as a percent of chord. As air flows over an inboard section of the unmodified Learjet wing having this particular airfoil type and moving at transonic speeds, the air quickly accelerates and the pressure coefficient tends to exceed the critical negative value over the frontal portion of the wing, indicating that the air has achieved supersonic velocities in this area. The coefficient of pressure then becomes less negative and actually goes positive over a generally chord-wise middle section of the wing. The air then again picks up speed, causing the coefficient of pressure to achieve a high, though sub-critical, negative value just prior to entry into the channel. The higher the negative coefficient of pressure, the greater the speed of the air, while a positive coefficient of pressure indicates relatively low speed air flow. With regard to the existing Learjet type wing as derived from NACA 64A-109 airfoil section, the air is caused to increase speed dramatically over the frontal and aft portions of the wing, while drastically reducing speed over the middle wing portion. As a result, the desirable lifting force indicated by the negative pressure coefficients is adversely affected by the positive or downward force generated in the area generally forward of the nacelle and indicated by the less negative and positive coefficients of pressure.
In comparison, applicant's invention includes a modified inboard portion of the airfoil which functions to continuously decelerate the air flowing over the wing in a substantially smooth manner from the high speed flow over the frontal portion of the wing to a much slower speed flow over the remaining portion of the wing, thereby helping ensure that the flow entering the channel remains essentially subsonic. The unique shape of the modified inboard wing also completely eliminates the undesirable downwardly directed force which adversely affects unmodified aircraft of this type.
A further problem facing aircraft of the unmodified Learjet type is recompression or rejoining of the downstream air after passage of the aircraft therethrough. It is considered desirable to leave the downstream air in a completely undisturbed state to minimize the profile drag generated by turbulent air flow. In the Gates Learjet type aircraft, a negative net pressure imbalance exists across a trailing portion of the nacelle pylon which generates increased drag, while also providing a resultant upward pressure against the pylon, which can adversely affect the force generated by the horizontal stabilizer.
Applicant's invention conveniently overcomes the problems of recompression by extensively modifying the top and bottom surfaces of the nacelle pylon, whereby a substantially balanced coefficient of pressure of between 0 and +0.1 Cp is generated over substantially the trailing 30% of the modified pylon structure. The substantially balanced pressure forces as indicated by the low net Cp value, function to reduce the drag at flight speeds of up to M=0.82, ensuring maximum thrust recovery. Furthermore, because the Cp values remain positive, adverse pressure on the horizontal stabilizer is eliminated.
As will be discussed in detail hereafter, applicant's new and useful invention solves the aforestated problems as well as additional problems confronting known Learjet type aircraft, while at the same time providing an aircraft aerodynamically capable of cruising at high altitudes in excess of 51,000 feet, and reducing drag generated during transonic flight (i.e. 0.75.ltoreq.M.gtoreq.0.80) by up to 17% as compared to an unmodified Learjet type aircraft operating under similar conditions.